Polluted Runoff Archives

Combined sewer overflow: how stormwater became Puget Sound’s #1 source of pollution

June 25, 2019 / ECOSS / No Comments

Stormwater runoff is the #1 source of pollution in Puget Sound, threatening human, fish and aquatic life. But how does rain result in pollution? One contributor is in how cities manage their sewage.

Seattle implemented its first centralized sewer system in the early 1900s ahead of a premier world fair. Rather than having separate pipes for sewage and stormwater, Seattle’s chosen system collected both sewage and stormwater through the same pipe. Generally, there was no issue as the combined sewage was treated at water treatment plants.

Graphic detailing the route of sewage under dry conditions.

Sewage is directed to a water treatment plant during dry weather.

However, heavy rain overloaded this system, dumping an overflow of untreated sewage and stormwater into Puget Sound, the Duwamish River and other local waters. The combined sewage included human waste, heavy metals from roads and pollutants accumulated on roofs. The polluted water is highly toxic to salmon, orcas and other aquatic wildlife. Moreover, it is detrimental to people swimming in the waters or fishing in urban rivers.

Graphic detailing the route of combined sewage during heavy storms.

During large storms, stormwater is combined with sewage in the same pipe, and the increased volume overflows straight into nearby waters.

The archaic combined sewer systems still exist today. Construction of new combined sewer systems stopped in the 1950s, but older systems have yet to be completely replaced. And with population growth and urbanization in Puget Sound, combined sewer overflow has only become more polluted.

Green Stormwater Infrastructure (GSI) such as rain gardens and cisterns reduce the amount of stormwater entering the combined sewers. More combined sewage is captured by the sewer system and treated properly, rather than going into Puget Sound waters.

Graphic detailing the route of combined sewage during heavy storms controlled by Green Stormwater Infrastructure (GSI).

Green Stormwater Infrastructure (GSI) manages stormwater so that sewer systems are less likely to overflow.

ECOSS engages multicultural communities to raise awareness about GSI opportunities for property owners so that communities of color can be included in the solutions for clean water and healthier communities. Furthermore, ECOSS recruits and guides multicultural contractors through training to install cisterns and rain gardens. These green career pathways promote small businesses of color and ensure that the benefits of GSI solutions are reaching Puget Sound’s most vulnerable communities.